The present invention relates to the field of opto-electronic or photonic devices, including photon-absorption and photon-emission processes. U.S. patent application Ser. No. 10/023,430 discloses an opto-electronic device architecture that can be implemented in different materials systems, including silicon and germanium. The essential features of that device architecture are the fabrication of an epitaxial film stack composed of alternating layers of a semiconductor (e.g. Si or Ge) and materials with wide band-gaps that can be made pseudomorphic with respect to the lattice parameter of the semiconductor (Si or Ge). Stacked Quantum Wells (QWs) and short-period SuperLattices (SLs) can be fabricated, having opto-electronic properties unavailable with the same materials in the “bulk” form. The thickness of the insulator and of the semiconductor layers control some very important electronic and photonic properties of the layer stack.
In that device architecture, the wavelength of light (emitted or absorbed) is determined by the energy levels of the QWs or SLs, which are fixed by the thickness of the relevant layers, and therefore are fixed during fabrication of said layers. For certain applications, such as “Dense Wavelength Division Multiplexing” (DWDM), it would be highly advantageous to be able to emit and/or absorb photons with wavelengths within fairly narrow intervals. That can be done in the original architecture by providing one or more active layers (QWs and/or SLs) for each of the wavelengths. However, this solution becomes impractical as the number of wavelengths increases.
One major feature of that device architecture is the provision of lateral contacts which are also band-gap engineered. Band-gap engineering in the vertical direction is common to many opto-electronic devices in several materials systems, but the band-gap engineering of the lateral contacts, as a method of selectively injecting and/or extracting charge carriers into particular sub-bands is was a novel concept.
The fabrication of different sets of lateral contacts to identical quantum wells, determine what kind of carriers are injected and/or extracted at each contact, and thus determine the type of opto-electronic processes to take place inside the quantum well. The architecture allows for several quantum wells (with different sub-bands) to be stacked upon each other, and to have different lateral contacts to each of the quantum wells, or to have some contacts in parallel to several quantum wells.
The band-gap engineering of the lateral contacts is crucial to ensure the proper operation of the device. This is especially true for the case of intersubband transitions. The original concept of the device architecture described in the patent application noted above includes a strategy to perform the energy filtering at the contacts through the careful alignment of the CB or VB edge of insulator materials and the work-function of conductors (metals, or metal suicides, or highly doped semiconductors). Unfortunately, the solution suggested in that patent application does not guarantee the desirable filtering for all carriers in all circumstances. For example, for a carrier traveling along the x-axis, when it meets an energy barrier also along the x-axis, the question of whether the carrier travels over that barrier is entirely dependent on its kinetic energy along the x-axis, and is completely independent from its energy along the other two axes.